Naturally Occurring A51V Variant of Human Cytochrome c Destabilizes the Native State and Enhances Peroxidase Activity

• Published in: The journal of physical chemistry. B Vol. 123; no. 42; pp. 8939 - 8953
• Main Authors: Lei, Haotian, Bowler, Bruce E
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 24.10.2019
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/acs.jpcb.9b05869

The A51V variant of human cytochrome c is linked to thrombocytopenia 4 (THC4), a condition that causes decreased blood platelet counts. A 1.82 Å structure of the A51V variant shows only minor changes in tertiary structure relative to the wild-type (WT) protein. Guanidine hydrochloride denaturation demonstrates that the global stability of the A51V variant is 1.3 kcal/mol less than that of the WT protein. The midpoint pH, pH1/2, of the alkaline transition of the A51V variant is 1 unit less than that of the WT protein. Stopped-flow pH jump experiments show that the A51V substitution affects the triggering ionization for one of two kinetically distinguishable alkaline conformers and enhances the accessibility of a high-spin heme transient. The pH1/2 for acid unfolding of the A51V variant is 0.7 units higher than for that of the WT protein. Consistent with the greater accessibility of non-native conformers for the A51V variant, the k cat values for its peroxidase activity increase by 6- to 15-fold in the pH range of 5–8 versus those of the WT protein. These data along with previously reported data for the other THC4-linked variants, G41S and Y48H, underscore the role of Ω-loop C (residues 40–57) in modulating the peroxidase activity of cytochrome c early in apoptosis.